A radio communication system such as an inter-automobile communication system is known in which direct communication is carried out between automobile communication apparatuses without any intervention of a base station. In this radio communication system, the communication state changes with movement of the communication apparatus. For example, with the movement of the communication apparatuses, it would be assumed that a communication are has no communication counter end or few communication counter end, or on the contrary, the communication apparatuses are dense in a certain area, in which there are a large number of communicable counter ends.
In particular, when the communication apparatuses are dense, radio communication is congested, resulting in the generation of congestion and the drop of throughput. At this time, when the communication is carried out through a base station, the base station controls the transmission power. Thus, the communicable area can be adjusted, which can equivalently suppress the number of the communication apparatuses in the communication area. Therefore, it is possible to reserve radio links with peripheral communication apparatuses, and to relax the generation of the congestion and to eliminate the drop of throughput. However, in the radio communication system in which the direct communication is carried out between the automobile communication apparatuses without any intervention of the base station, such as the inter-automobile communication system, each automobile communication apparatus carries out the direct communication. Thus, it is difficult to properly set the transmission powers of those automobile communication devices. Each automobile communication apparatus is expected to autonomously set the proper transmission power on the basis of the distribution of the peripheral communication apparatuses.
Japanese Patent Application Publication (JP-P2003-143048A) discloses an inter-automobile communication apparatus and an inter-automobile communication method. The inter-automobile communication apparatus exchanges data through a radio communication between a plurality of automobiles. The inter-automobile communication apparatus retrieves peripheral automobiles that can relay a transmission data to a target automobile, in accordance with the communication state of the target automobile as a counter end to which the data is transmitted. Then, the inter-automobile communication apparatus has a section that wirelessly transmits the transmission data to the target automobile through a relaying automobile, if the relaying automobile that can carry out the relaying operation is retrieved. That is, this inter-automobile communication apparatus can carry out the transmission/reception without any increase in the transmission power, by changing the transmission path of the transmission data in accordance with a reception level of reception data.
Japanese Patent Application Publication (JP-P2003-258715A) describes a radio communication apparatus. This radio communication apparatus is installed in an automobile and used for the inter-automobile radio communication. This radio communication apparatus detects the number of automobiles in the vicinity of the automobile itself and randomly determines a transmission interval in accordance with the number of automobiles, and then transmits a frame including the vehicle information in the transmission interval. Also, there is a case that this radio communication apparatus randomly determines a transmission interval in accordance with the running speed corresponding to the running speed data of the automobile itself supplied from outside and transmits the frame including the vehicle information in the transmission interval.